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G Protein-Coupled Receptors & Second Messengers

The workhorse of fast hormone signaling. A GPCR catches a hormone outside and flips a switch inside, releasing a flood of small molecules — cyclic AMP, IP3, DAG, calcium — that carry the message onward. Learn the three main relay lines and what each one tells the cell to do.

The most common receptor design in the body

A G protein-coupled receptor (GPCR) is a single protein that snakes through the membrane seven times. The hormone binds on the outside; the inside end is coupled to a G protein, a molecular switch that sits ready in the membrane. Hundreds of hormones and neurotransmitters use this one design, which is why GPCRs are the single largest receptor family and the target of a huge share of medicines.

  1. The hormone binds the outside of the GPCR, changing its shape.
  2. The activated receptor nudges its G protein, which swaps an old GDP for a fresh GTP and turns ON.
  3. The G protein splits off and switches on (or off) an enzyme that makes a second messenger.
  4. The second messenger spreads through the cell and triggers the response. After a moment, the G protein's own timer turns it off.

Relay line 1: cyclic AMP

The classic line. A stimulating G protein switches on adenylyl cyclase, the enzyme that converts ATP into cyclic AMP (cAMP). Cyclic AMP then activates protein kinase A (PKA), an enzyme that tags other proteins with phosphate groups, switching them on or off. This is how adrenaline tells the liver to release glucose and how many pituitary and adrenal hormones act.

Crucially, GPCRs come in stimulating and inhibiting flavors. An inhibiting G protein lowers cAMP instead of raising it. So the very same messenger pool can be pushed up by one hormone and pulled down by another — the cell reads the balance, not just one signal.

Relay line 2: IP3, DAG, and calcium

A different G protein switches on an enzyme that splits a membrane lipid into two messengers at once: IP3 and diacylglycerol (DAG). IP3 floats into the cell and opens calcium gates on internal stores, so calcium floods the cytoplasm. DAG stays in the membrane and switches on protein kinase C. Calcium itself is a powerful messenger — it triggers muscle contraction, secretion of stored hormones, and many enzymes.

GPCR    -> stimulating G protein -> adenylyl cyclase -> cAMP -> PKA
GPCR    -> inhibiting  G protein -> LESS cAMP
GPCR    -> Gq protein   -> phospholipase C -> IP3 + DAG
                                            |        |
                                            v        v
                                  release Ca2+   activate PKC

(One receptor family, three messages, opposite directions possible.)
The three GPCR relay lines. Notice the same architecture produces up, down, and a branching calcium signal.